1. Field of the Invention
The invention relates generally to the determination of orientation heading through sensing Earth's magnetic field, and more particularly to a method and an apparatus to correct compass magnetic heading indications.
2. Description of Related Art
U.S. Pat. No. 5,525,901 describes a sensor system for monitoring and measuring angular position in two or three axes; the patent teaches how to measure angular position in three axes with two orthogonal magnetoresistive sensors and three amplifiers and an external biasing magnetic field. Currently Clymer Technologies offers a product, Terrella6™, a heading and motion sensor capable of determining roll, pitch, yaw, translational accelerations, angular rates, and compass heading; it is reportedly a sealed device containing no moving parts and has interference detection to reduce ferrous metal disturbances. U.S. Pat. No. 6,836,971 discloses a system for using a 2-axis magnetic sensor for a 3-axis compass solution using a stored value of the earth's magnetic field applicable to a geographic region. U.S. Pat. No. 7,069,663 discloses an apparatus and method for calibrating azimuth of a mobile device utilizing a calibration parameter. U.S. Pat. No. 7,162,807 discloses a geomagnetic sensor supporting a dip angle detection function with orthogonal fluxgate sensors and orthogonal acceleration sensors.
As used herein magnetic field sensor technologies include nuclear precession, optically pumped, and fiber-optic magnetometers. The magnetic range for the medium field sensors lends itself well to using the Earth's magnetic field. Several ways to use the Earth's field are to determine compass headings for navigation, detect anomalies in it for vehicle detection, and measure a derivative of the change in field to determine yaw rate. Fluxgate magnetometers are the most widely used sensor for compass navigation systems. Fluxgate sensors have also been used for geophysical prospecting and airborne magnetic field mapping. The most common type of fluxgate magnetometer is called the second harmonic device. This device involves two coils, a primary and a secondary, wrapped around a common high-permeability ferromagnetic core. A way of looking at the fluxgate operating principle is to sense the ease, or resistance, of saturating the core caused by the change in its magnetic flux. The difference is due to the external magnetic field. Fluxgate magnetometers can sense signal in the tens of micro-gauss range. Fluxgates can measure both magnitude and direction of static magnetic fields. A magnetoinductive magnetometer comprises a sensor with a single winding coil on a ferromagnetic core that changes permeability within the Earth's field. The sense coil is the inductance element in a L/R relaxation oscillator. The frequency of the oscillator is proportional to the field being measured. A static dc current is used to bias the coil in a linear region of operation. The observed frequency shift can be as much as 100% as the sensor is rotated 90 degrees from the applied magnetic field. The oscillator frequency can be monitored by a microprocessor's capture/compare port to determine field values. William Thompson first observed the magnetoresistive effect in ferromagnetic metals in 1856. Magnetoresistive (MR) sensors come in a variety of shapes and form; common applications include disk drive heads, automotive wheel speed and crankshaft sensing, compass navigation, vehicle detection, current sensing, and many others. An anisotropic magnetoresistive (AMR) sensor is one type that lends itself well to the Earth's field sensing range. AMR sensors can sense dc static fields as well as the strength and direction of the field. Frequently, this sensor is made of a nickel-iron thin film deposited on a silicon wafer and is patterned as a resistive strip. The properties of the AMR thin film cause it to change resistance by 2-3% in the presence of a magnetic field. The reaction of the magnetoresistive effect is very fast and not limited by coils or oscillating frequencies.
The Earth's magnetic field intensity is about 0.5 to 0.6 gauss and has a component parallel to the Earth's surface that always point toward magnetic north. This is the basis for all magnetic compasses. The Earth's magnetic field can be approximated with a dipole model; the Earth's magnetic field points down toward north in the northern hemisphere, is horizontal and pointing north at the equator, and point up toward north in the southern hemisphere. In all cases, the direction of the Earth's field is always pointing to magnetic north. It is the components of this field that are parallel to the Earth's surface that are used to determine compass direction. The vertical portion of the Earth's magnetic field is frequently ignored.
Most often compasses are not confined to a flat and level plane; they are often hand held, attached to an aircraft, or on a vehicle in an uneven terrain. This makes it more difficult to determine an azimuth, or heading direction, since a compass is not always horizontal to the Earth's surface. Errors introduced by tilt angles can be quite large depending on the amount of the Dip angle. A typical method for correcting a compass tilt is to use an inclinometer, or tilt sensor, to determine the roll and pitch angles. The terms roll and pitch are commonly used in aviation: roll refers to the rotation around the X, or forward direction, and pitch refers to the rotation around the y, or left-right, direction. Liquid filled tilt sensors, resembling a glass “thimble”, use electrodes to monitor the fluid movement as the sensor changes angles. Solid state accelerometer tilt sensors are available that measure the Earth's gravitational field by means of an electromechanical circuit or MEMS based accelerometers are also available. The output of these devices can be an electrical signal equivalent to the angle of tilt.
To achieve a one degree accurate compass requires a magnetic sensor that can reliably resolve angular changes to 0.1 degrees. Magnetic sensors must also exhibit low hysteresis (<0.05% FS), a high degree of linearity (<0.5% FS error) and be repeatable. Magnetic fields in the X and Y plane are typically in the 200 to 300 milli-gauss range—more at the equator, less at the poles. To resolve a 0.18 degree change in a 200 milli-gauss field requires a magnetic sensitivity of better than 0.35 milli-gauss. To compensate a compass for tilt, knowing the roll and pitch is required as well as other factors. Historically, magnetometers required all magnetic axes (X, Y, Z) so that the Earth's field is easily fully rotated back to a horizontal orientation. A compass has roll (r) and pitch (p) tilt angles referenced to the right and forward level directions of an observer or vehicle. The X, Y, and Z magnetic readings are transformed back to a horizontal plane (XH, YH) by applying rotational equations. Once the X and Y magnetic readings are in a horizontal plane, equations can be used to determine an azimuth. A block diagram for a tilt compensated compass of the instant invention is shown in FIG. 4. After an azimuth is determined, a declination correction is applied to find true north according to the geographic region of operation.